The robotic mini-submarine deployed to the Indian Ocean floor to hopefully find Malaysia Flight 370 made a premature return to the surface today after exceeding its maximum depth.

The Bluefin 21 is not supposed to dive any deeper than 4,500 meters (roughly 14,800 feet or 2.8 miles) below the surface. Unfortunately, the area the possible black box pings were heard is up to three miles deep. When the submarine exceeded its depth limit after six hours, a safety feature kicked in and returned it to the surface. The submarine was supposed to be down there for 16 hours, covering a 15.5 square mile area.

The search committee said the information the submarine did manage to get during its abbreviated trip is being analyzed.

The submarine is expected to be deployed again tomorrow, weather permitting. In the meantime, search crews are also investigating an oil slick seen near the ping detection site. It could be from the missing plane, or it could be from, well, any of the other air and sea vehicles involved in the search. A sample of the oil is being tested, but an expert told NBC News there isn't any way to link the oil to a specific plane -- they all use the same jet fuel.

The submarine is one of our last best hopes for finding the plane; the black box's batteries are probably dead, and pings haven't been heard in days. The visual search is scheduled to end in a few days.

This article is from the archive of our partner The Wire.

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